As glaciers moved across the land, rock and sediment were eroded, carried, and blended to create a mixture of boulders, sand, and clay known as till. As the glaciers receded, tills were revealed in depths to tens of metres.

Over time, percolating water, chemical reactions, and biological activity created soil horizons within the till. Plants decayed to form an organic or humus-rich layer of top soil. Humus adds valuable nutrients to the weathered till to produce excellent soil for agriculture.

Till is the predominant parent or starting material for soil in the GTA.

The Canada Land Inventory ranks agricultural potential using soil depth, drainage, water-holding capacity, and fertility as well as slope of the land, climate, length of growing season, and susceptibility to erosion. Based on this classification only 5% of Canada is prime agricultural land. Class 1 to 3 soils are considered to be prime agricultural land and almost 80% of the GTA was originally covered by soils of these classes.

Organic soils, like those in the Holland Marsh north of Newmarket, consist of decayed plant matter that accumulated in wetlands that were previously glacial meltwater ponds or channels. When cleared of their natural vegetation, these mucky black soils are used to grow specialty crops such as vegetables. Should they be cleared for agriculture or retained as wetlands to filter water and provide wildlife habitat?

Agriculture is second only to the auto industry in Ontario's economy. In the GTA alone, agriculture is a $1.3 billion industry with farm receipts totalling about $600 million and indirect sales generating another $700 million annually (1996 Census).

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